1. Field of the Invention
The present invention relates to a method of manufacturing a carcass structure for tires for vehicle wheels.
The invention also relates to a carcass structure for tires for vehicle wheels obtainable by the above method, said carcass structure comprising: at least one carcass ply formed of strip-like sections each of which extends in a substantially U-shaped conformation following the cross-section outline of the tire and comprises one or more thread-like elements longitudinally arranged parallelly of each other and preferably at least partly coated with a layer of raw elastomer material; and a pair of annular reinforcing structures each engaged close to a respective inner circumferential edge of the carcass ply.
2. Description of the Related Art
Manufacture of tires for vehicle wheels involves formation of a carcass structure essentially made up of one or more carcass plies substantially having a toroidal conformation and presenting their axially opposite side edges engaged to respective annular, circumferentially inextensible, reinforcing elements usually referred as xe2x80x9cbead coresxe2x80x9d.
Applied to the carcass structure, at a circumferentially outer position, is a belt structure comprising one or more belt strips having the shape of a closed ring, which are essentially made up of textile or metal cords suitably oriented relative to each other and to the cords belonging to the adjacent carcass plies.
Then, at a circumferentially outer position of the belt structure a tread band is applied which usually consists of a strip of elastomer material of suitable thickness.
It is to point out that, to the aims of the present description, by the term xe2x80x9celastomer materialxe2x80x9d it is intended a rubber blend in its entirety, i.e. the assembly formed of a base polymer suitably amalgamated with mineral fillers and/or additives of other type.
Ultimately, a pair of sidewalls is applied to the opposite sides of a tire being manufactured, each of said sidewalls covering a side portion of the tire included between a so-called shoulder region, arranged close to the corresponding side edge of the tread band, and a so-called bead arranged at the corresponding bead core.
The traditional production methods essentially provide that the above listed tire components should be first made separately of each other, to be then assembled during a step of the tire manufacturing.
For example, for making the carcass ply or plies to be associated with the bead cores in order to form the carcass structure, it is first required that, by an extrusion and/or calendering process, a rubberized fabric comprising longitudinally-disposed continuous textile or metal cords should be produced. This rubberized fabric is submitted to a transverse cutting operation to produce sections of predetermined lengths that are subsequently joined together so as to give origin to a continuous ribbon-like semifinished product, having transversely-arranged parallel cords.
This manufactured article must then be cut into sections the length of which is to be correlated with the circumferential extension of the carcass to be produced.
Also proposed have been manufacturing methods that, instead of resorting to production of semifinished articles, make the carcass structure directly during the tire manufacturing steps.
For example, U.S. Pat. No. 5,453,140 herein referred to as an example of the most pertinent state of the art, discloses a method and an apparatus forming the carcass ply starting from a single cord that has been previously wound up on a reel.
In accordance with the method and apparatus described in the above patent, at each working cycle of the apparatus the cord taken up from the reel by power-driven pulling rollers and maintained taut by a pneumatic tensioning system is cut to size so as to obtain a section of a predetermined length.
The cord section is taken up by a grip element mounted on a belt looped around power-driven pulleys to be transversely laid down on the outer surface of a toroidal support.
The section ends are then engaged by folding members of the belt type operating on opposite sides of the toroidal support to radially apply the cord section to the toroidal support itself by slider elements acting like fingers along the section side portions.
Repetition of the above described working cycle leads to deposition of these cord sections in a circumferential side-by-side relationship until the whole circumferential extension of the toroidal support is covered.
Necessarily, the toroidal support is previously coated with one or more raw rubber layers having a dual function, that of adhering to the cords laid down thereon so as to conveniently hold them in a fixed positioning, and that of forming an inner air-proof layer in the finished tire.
Tires obtained by this manufacturing method have a carcass structure in which the carcass ply or plies are formed of individual cords each having two side portions axially spaced apart from each other and radially oriented relative to the rotation axis of the tire, and a crown portion extending at a radially outer position between the side portions.
Within the scope of the carcass structure manufacture, it is also known that close to each of the tire beads, the opposite ends of the individual cords forming a carcass ply are arranged in an alternating sequence, at axially opposite positions relative to an annular anchoring element forming said bead core, having the shape of a crown made up of radially-overlapping wire coils, as can be viewed from Patent EP 0 664 231 and U.S. Pat. No. 5,702,548.
However, the cords forming the carcass ply or plies are substantially arranged in the neutral axis of resistance to bending of the respective bead. Under this circumstance, the structural resistance of the beads must necessarily rely on the stiffness of the filling inserts of very hard elastomer material incorporated into the bead structure, the behaviour of which feels the effects of temperature changes due both to environmental factors and to stresses produced during normal operation.
In Patent FR 384 231 it is proposed to make a carcass structure by deposition on a toroidal support, of a series of rectangular small bands of rubberized fabric disposed in side-by-side relationship and arranged in radial planes relative to the geometric axis of the support drum itself. Deposition of the small bands is carried out in such a manner that the end flaps of two non-consecutive small bands are partly covered by the end flaps of the small band interposed therebetween. Spaces existing between the end flaps of the covered small bands are filled with trapezoidal inserts applied to the end flaps of the small band placed in overlapping relationship thereon. Deposition of the small bands is executed in different overlapped layers, the number of which is correlated with the thickness to be given to the carcass structure. The presence of said trapezoidal inserts gives rise to thickening of the carcass structure at the bead regions, where it has a thickness which is twice that found at the crown.
In U.S. Pat. No. 4,248,287 it is disclosed a method according to which formation of the carcass structure involves that a plurality of layers each formed of radial strips consisting of rubberized threads and circumferentially disposed in side by side relationship should be laid down on a toroidal drum. When deposition has been completed, at the bead region two bead cores are applied and the end flaps of the carcass layers formed by the radial strips are then turned back around them.
In accordance with the present invention, it has been found that within the tire-manufacturing scope surprising advantages can be achieved if the carcass ply or plies are made by laying down at least two distinct series of strip-like sections in an alternating sequence, and arranging the bead-reinforcing annular structures, or at least part of them, at an axially interposed position between the end flaps belonging to the sections of one series and those of the other series, respectively.
In more detail, the invention relates to a method of manufacturing a carcass structure for motor-vehicle tires, characterized in that it comprises the steps of: preparing strip-like sections each comprising one or more longitudinal and parallel thread-like elements preferably coated at least partly with at least one layer of raw elastomer material; laying down a first series of said strip-like sections onto a toroidal support, each of them extending in a substantially U-shaped configuration around the cross-section outline of the toroidal support itself and circumferentially distributed according to a circumferential pitch corresponding to a multiple of the strip-like section width, applying at least primary portions of annular reinforcing structures against end flaps of said strip-like sections belonging to the first series, at axially opposite positions relative to an equatorial plane of the support drum; laying down on the toroidal support at least one second series of said strip-like sections each extending according to a U-shaped conformation around the cross-section outline of the toroidal support, between two consecutive sections of the first series, each of the sections of the second series having end flaps overlapping the respective primary portions of the annular reinforcing structures at an axially opposite position relative to the end flaps of the sections of the first series.
Preferably, also carried out is the step of applying additional portions of the annular reinforcing structures against the end flaps of the strip-like sections belonging to the second series, so that each of said end flaps is enclosed between the primary portion and the additional portion of the respective annular reinforcing structure.
It is also preferably provided that each of the strip-like sections of the first and second series should be laid down so as to form two side portions substantially extending in the direction of the geometric rotation axis of the toroidal support at mutually spaced apart positions in an axial direction, and a crown portion extending at a radially outer position between the side portions, the crown portions of each strip-like section being disposed consecutively in side by side relationship along the circumferential extension of the toroidal support.
It can be also provided that the side portions of each strip-like section belonging to the first series should be each partly covered with a side portion of at least one circumferentially consecutive section belonging to the second series, at a stretch included between a radially outer edge of the primary portion of the annular reinforcing structure and a transition region between said side portions and said crown portions.
In more detail, covering of the side portions of each strip-like section belonging to the first series progressively decreases starting from a maximum value close to the outer circumferential edge of the primary portion of each annular reinforcing structure until a null value at the transition regions between said side portions and crown portions.
Preferably, the side portions of said strip-like sections are made radially converge towards the geometric rotation axis of the toroidal support.
Also provided may be at least one operating step aiming at defining regions of greater width close to the inner circumferential edges of the carcass structure.
If, as provided in a preferential embodiment, preparing of said strip-like sections takes place by cutting actions sequentially carried out on at least one continuous strip-like element incorporating said thread-like elements in said layer of raw elastomer material, the step of defining regions of greater width can be advantageously carried out on the continuous strip-like element before performing the cutting action.
In accordance with a further aspect of the invention, to be also adopted independently of that which has been previously said, accomplishment of said at least one primary portion of each annular reinforcing structure comprises the steps of: laying down at least one first elongated element in concentric coils to form a first circumferentially inextensible annular insert substantially in the form of a crown; forming at least one filling body of raw elastomer material; joining the filling body to the first circumferentially inextensible annular insert.
In more detail, the first elongated element is preferably deposited directly against the end flaps of the strip-like sections belonging to the first series, to form the first annular insert directly in contact with the strip-like sections themselves.
The filling body can be in turn formed by depositing a continuous strip of elastomer material directly against the first annular insert previously applied to the end flaps of the strip-like sections belonging to the first series.
In accordance with a further embodiment, the first elongated element is laid down in a forming seat defined in a moulding cavity in which the filling body is subsequently formed, so that joining of said filling body to the first annular insert is carried out concurrently with formation of the filling body itself.
Alternatively, the joining step can be carried out by applying the filling body against the first annular insert previously applied to the end flaps of the sections belonging to the first series.
It can be also provided that formation of said primary portion of the annular reinforcing structure should comprise the further steps of: depositing at least one second elongated element in concentric coils to form a second circumferentially inextensible annular insert substantially in the form of a crown; and joining said second annular insert to the filling body, on the opposite side relative to the first annular insert.
Preferably, formation of said additional portion of each annular reinforcing structure comprises the step of depositing at least one second elongated element in concentric coils to form a second circumferentially inextensible annular insert substantially in the form of a crown.
Said second elongated element is preferentially laid down directly against the carcass structure during a formation step on the toroidal support.
It is a further object of the invention to provide a carcass structure for vehicle wheel tires, characterized in that said carcass ply comprises: a first and a second series of strip-like sections arranged in a mutually alternating sequence along the circumferential extension of the carcass structure, each of said annular reinforcing structures comprising at least one primary portion having an axially inner side turned towards the end flaps of the sections belonging to the first series and an axially outer side turned towards the end flaps of the sections belonging to the second series.
It is preferably provided that each of said annular reinforcing structures further comprises at least one additional portion disposed against the end flaps of the strip-like sections belonging to the second series, on the opposite side relative to the primary portion of the annular structure itself.
Preferably, each of said strip-like sections has two side portions substantially extending in the direction of the geometric rotation axis of the carcass structure at mutually spaced apart positions in an axial direction, and a crown portion extending at a radially outer position between the side portions, the crown portions belonging to the sections of the first and second series respectively being arranged in side by side relationship along the circumferential extension of the carcass structure.
It can be also provided that the side portions of each strip-like section belonging to the first series should be each partly covered with a side portion of at least one adjacent strip-like section belonging to the second series, at a stretch included between a radially outer edge of the primary portion of the annular reinforcing structure and a transition region between said side portions and crown portions.
In more detail, covering of the side portions of each strip-like section belonging to the first series progressively decreases starting from a maximum value close to the outer circumferential edge of the primary portion of each annular reinforcing structure until a null value at the transition regions between said side portions and crown portions.
Advantageously, the side portions of said strip-like sections radially converge towards the geometric rotation axis of the carcass structure.
The individual strip-like sections belonging to one of said first and second series respectively are advantageously disposed according to a circumferential distribution pitch corresponding to a multiple of the width of the strip-like sections themselves.
It can be also provided that each strip-like section should have regions of greater width close to the inner circumferential edges of the carcass structure.
In this case, the thread-like elements included in each strip-like element are mutually spaced apart at said regions of greater width.
Preferably, each of said strip-like sections has a width included between 3 and 15 mm, and comprises three to eight thread-like elements.
In particular, said thread-like elements are disposed in the respective strip-like sections according to a mutual distance between centres not lower than 1.5 times the diameter of the thread-like elements themselves.
In accordance with a further independent aspect of the invention, the primary portion of each of said inextensible annular structures comprises: a first circumferentially inextensible annular insert substantially in the form of a crown disposed coaxially with the carcass structure and close to an inner circumferential edge of the first carcass ply, said first annular insert being made up of at least one elongated element extending in concentric coils; and a filling body of elastomer material having a side joined to the first annular insert.
It can be also provided that each of said annular reinforcing structures should further comprise at least one second circumferentially inextensible annular insert substantially in the form of a crown, made up of at least one elongated element extending in concentric coils and disposed coaxially with the carcass structure at a position axially in side by side relationship with the filling body and laterally opposite to the first annular insert.
Preferably, said second annular insert is part of an additional portion of said reinforcing structure disposed against the end flaps of the strip-like sections belonging to the second series, on the opposite side relative to the primary portion of the annular structure itself.
Conveniently the second circumferentially inextensible annular insert has a radial extension greater than the radial extension of the first circumferentially inextensible annular insert, and said filling body of elastomer material has a hardness included between 48xc2x0 and 55xc2x0 Shore D at 23xc2x0 C.